Vesicular transport is the general process by which proteins synthesized in the endoplasmic reticulum are transported via the Golgi network to their appropriate cellular location, either at the cell surface or within various membrane bound organelles within the cytosol. Many of the proteins involved in this complex process are conserved among all eukaryotes.
The transport pathway involves formation of a series of transport vesicles that shuttle proteins from one membrane-bound compartment to another until they reach their final destination. Sorting of proteins into the appropriate transport vesicle is mediated by specific receptors in the golgi which recognize the proteins and sequester them into transport vesicles. For example, proteins destined for the mammalian lysosome, equivalent to the yeast vacuole, are tagged in the golgi with mannose-6-phosphate (Man-6-P) moieties. This tag is recognized in the trans golgi network by Man-6-P receptors (MPRs) which mediate the sorting and transport of lysosomal proteins to an endosomal compartment. The proteins are subsequently delivered to a lysosome by an unknown mechanism.
Recognition between donor and acceptor compartments is mediated by snare proteins, complementary identifiers displayed on the surface of transport vesicles (v-snares) and their appropriate target compartments (t-snares). Following recognition between complementary snare proteins, the two membrane bound compartments fuse to complete the delivery of the protein. The receptors are then recycled back to the golgi for another round or sorting and transport. Proteins that do not carry targeting signals for delivery to an intracellular compartment, such as constitutively secreted proteins and plasma membrane proteins, instead enter a default pathway (bulk flow) in which they are sorted into vesicles that eventually fuse with the plasma membrane. (Rothman, J. E. and Wieland, F. T. et al. (1996) 727:227-33.) Components of the vacuolar protein sorting machinery have been identified in yeast through a variety of genetic selection schemes. A set of vacuolar protein sorting (vps) mutants have defined at least 45 genes that encode proteins involved in vesicular transport. One of these genes, Vps35, was identified in a selection for mutants that mislocalize and secrete a hybrid protein. This hybrid consisted of a fusion between a soluble vacuolar hydrolase, carboxypeptidase Y (CPY), and a secreted enzyme, invertase. Vps35 mutants also mislocalize and secrete endogenous CPY, but not other vacuolar proteins, e.g., proteinase A, proteinase B, and alkaline phosphatase. Vps35 mutants are viable and have morphologically normal vacuoles, indicating that they are competent in assembly of the vacuolar compartment. Thus, Vps35 mutants appear to disrupt an alternate pathway required for the sorting and/or transport of a subset of vacuolar proteins.
Mutants with a selective missorting phenotype similar to Vps35, such as Vps10, Vps29, and Vps30, define additional components of the Vps35 dependent sorting/transport pathway. Vps10 encodes a transmembrane protein that shuttles between golgi and vacuolar compartments, and functions as a receptor for soluble vacuolar hydrolases, such as CPY. Vps29, Vps30, and Vps35 encode novel hydrophilic proteins that all appear to be required for normal recycling of Vps10 receptors from the vacuole back to the golgi. Vps35 encodes a 110 kDa protein that binds directly to Vps10 and appears to associate peripherally with golgi membranes. In Vps29 or Vps30 mutants, both Vps10 and Vps35 are mislocalized to a cell fraction enriched in vacuoles. (Paravicini, G. ct al. (1992) Mol. Biol. Cell 3:415-427; Marcusson et al. (1994) Cell 77:579-586; and Seaman et al. (1997) J. Cell Biol. 137:79-92.)
A mammalian gene, Mem3, which shares 33% amino acid sequence identity with Vps35, has been isolated from a library of mouse genes highly expressed in unfertilized eggs. Mem3 is also expressed at high levels during preimplantation development and at lower levels in adult tissues. This expression pattern is consistent with a proposed role for Mem3 as a housekeeping protein involved in vesicular transport. (Paravicini et al., supra; Hwang, S. -Y. Et al. (1996) Mammalian Genome 7:586-590.)
Defects in the expression or function of components of vesicular transport pathways can result in mislocalization, and consequent abnormal function, of many critical proteins, e.g., membrane receptors, transporters and other membrane proteins, neurotransmitters, hormones, and lysosomal and digestive enzymes. Thus, components of vesicular transport pathways may play a play a role in certain disorders of cell proliferation and metabolism.
The discovery of a new human Vps35/Mem3-related protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cancer, inflammatory disorders, lysosomal storage diseases, and disorders of membrane transport.